In a supercritical heat pump cycle system described in WO 93/06423 (JP-B2-2931668), a refrigerant temperature at a refrigerant outlet of a refrigerant radiator is detected. Furthermore, an open degree of a decompression device is controlled based on the refrigerant temperature to control the pressure of high-pressure refrigerant, so that coefficient of performance (COP) of the refrigerant cycle becomes in maximum.
Generally, in a heating operation for heating air (fluid) using a refrigerant radiator, an air temperature heated by the refrigerant radiator, a temperature variation in air flowing from the refrigerant radiator, and an air temperature distribution are greatly affected to a comfortable performance of the heating. Furthermore, the refrigerant temperature at the outlet of the refrigerant radiator is greatly changed based on an air amount flowing in the refrigerant radiator, an air temperature drawn into the refrigerant radiator and a rotational speed of the compressor. Therefore, in the heating operation, the temperature of air flowing out of the refrigerant radiator is also greatly changed.
The refrigerant amount flowing to the refrigerant radiator can be controlled using a variable displacement compressor. However, when the temperature of suction air is low (e.g., 0° C.) in the heating operation, it is difficult to stably control the capacity of the variable displacement compressor. For example, when the refrigerant amount discharged from the compressor is controlled to be smaller by using the variable displacement compressor, the high-pressure refrigerant pressure decreases. In this case, if the open degree of the decompression device is controlled to be smaller in order to maintain the high-pressure refrigerant pressure, the refrigerant circulating amount is changed and the refrigerant discharge capacity is also changed. Accordingly, the high-pressure refrigerant pressure or the refrigerant discharge capacity of the compressor cannot be stably controlled.